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POHow to throttle concurrent Async webrequests
 

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  1. POHow to throttle concurrent Async webrequests
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    <p>I often need to make a large number of webrequests, without overloading the network</p> <p>I currently do this by running synchronous requests in parallel, utilizing ThreadPool.SetMinThreads and MaxDegreeOfParallelism to exactly specify how many requests run concurrently</p> <p>Now this works just fine, but it feels wrong.</p> <p>I would really like to utilize async methods, but i cant work out how to limit the number of concurrent requests.</p> <p>A simplified example of my parallel way of doing this( using a webclient and no error handling for brevity):</p> <pre><code>Private Function SearchSitesForKeywordInParallel(ByVal keyword As String, ByVal sites As String(), ByVal maxConcurrency As Integer) As String() Dim po As New ParallelOptions po.MaxDegreeOfParallelism = maxConcurrency Threading.ThreadPool.SetMinThreads(maxConcurrency, 2) Dim sitesContainingKeyword As New Concurrent.ConcurrentBag(Of String) Parallel.For(0, sites.Count, po, Sub(i) Dim wc As New Net.WebClient wc.Proxy = Nothing Dim pageSource As String = wc.DownloadString(sites(i)) If pageSource.Contains(keyword) Then sitesContainingKeyword.Add(sites(i)) End If End Sub) Return sitesContainingKeyword.ToArray End Function </code></pre> <p>This is a blocking function, which is what i require. Now i have tested the webclient.downloadStringAsync method in a regular for loop, and it will fire all the requests pretty much at once, overloading the network.</p> <p>What i would like to do is initially make X requests, then make new ones as each response comes back.</p> <p>I am fairly sure tasks is the way to go, and im positive a have read some very nice implementations in c#, but my c# experience is limited, and i have a hard time translating c# lambadas to vb.net.</p> <p>I am also limited to vs2010 and .net4, so the niceties of .net4.5 async await are not an option for me.</p> <p>Any help very much appreciated</p>
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    1. COCan you upgrade to VS2012 while still targeting .NET 4.0?
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    2. CO@StephenCleary sadly not, i am stuck with vs2010 for the foreseeable future.
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    3. COIn that case, I would stick with `MaxDegreeOfParallelism` or `SemaphoreSlim`. You *could* build your own asynchronous system (which would reduce the number of threads), but it would be excessively complex.
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